Nmlc ef4 module1 day03 corrosion and preventions

EXCELLENCE AND COMPETENCY TRAINING CENTER INC.
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NMLC-EF4-Module 1
Function 4:
Controlling the operation of
the ship and care for
persons onboard at the
management level.
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The prevention of corrosion on board ship is an immense
ongoing process demanding the attention and skills of
considerable numbers of personnel. The ship because of
its size, its physical environment and the materials used
in its construction is subject to attack from the various
forms of corrosion.
Corrosion and its Prevention

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Corrosion is the wasting of metals by chemical or
electrochemical reactions with their surroundings. Erosion
is a term often associated with corrosion and refers to the
destruction of a metal by abrasion. Erosion is therefore a
mechanical wastage process that exposes bare metal
which can then corrode.
Iron and steel corrode in an attempt to regain their oxide
form which is in a balanced state with the earth’s
atmosphere. This oxidizing, or rusting as it is commonly
termed, will take place whenever steel is exposed to
oxygen and moisture. The prevention of corrosion
therefore deals with the isolation of steel from its
environment in order to stop this oxidation taking place.
Corrosion

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In addition, the presence of a ship almost constantly in
sea water enables an electrochemical reaction to takes
place on unprotected steel surface. A corrosion cell is
then said to have been formed. This is often referred to
as a ‘galvanic cell’, since its current flow is a result of a
potential difference between two metals (not necessarily
different) in a solution such as sea water. This current
flow results in metal being removed from the anode
metal or positive electrode, while the cathodic metal or
negative electrode is protected from corrosion. Most
common metals can be arranged in what is known as a
galvanic series, according to their electrical potential in
sea water.

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Effects of Corrosion
A simple example of a corrosion cell would be a plate of
copper and one of iron placed in a sea water solution and
joined by a wire. The copper will become the cathode or
protected end and the steel will become anodic and
corrode.
Corrosion can also occur as a result of stress, either set
up in the material during manufacture or as a result of its
‘working’ in the sea. The effects of stress and fatigue are
to provide areas where cracking may occur, but even
these sometimes minute cracks create conditions under
which galvanic corrosion will proceed. The combined
action of the two has a considerable effect on the
material.

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The prevention of corrosion deals in the first place with
the provision of an adequate protective coating for the
ship’s structural steel and its continued maintenance.
Secondly, a means of preventing electrochemical wastage
is required, which is known as cathodic protection. The
two distinctly different types of corrosion prevention are
usually complementary to one another in that both are
normally fitted on modern ships. Finally, it should be
noted that knowledge of the processes of corrosion can
ensure the reduction or prevention of corrosion on board
ship, particularly on the internal structure, by the use of
good design and arrangement of structural members.
Corrosion prevention

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When a metal is in contact with an electrolyte, e.g. the
steel of a ship’s hull in sea water, small corrosion cells
may be set up due to slight variations in the electrical
potential of the metal’s surface. Electric currents flow
between the high and low potential points, with the result
that metal is corroded from the point where the current
leaves the metal (the anode). At the point where the
current re-enters the metal (the cathode) the metal is
protected. Cathodic protection operates by providing a
reverse current flow to that of the corrosive system. With
current then entering the metal at every point, the whole
metal surface becomes a cathode, and it is therefore
cathodically protected.
Cathodic protection

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When the potential over the immersed hull surface is
0.80-0.85 V more negative than a reference silver/silver
chloride electrode in the water nearby, then the hull is
adequately protected. Current density of the order of
20-100 mA/m2 is usually sufficient on a painted hull to
reverse any corrosion current and cease further metal
corrosion. Current density necessarily increases for a
poorly painted hull and therefore cathodic protection
should be regarded as an additional protection to painting
and by no means a substitute.

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When the potential over the immersed hull surface is
0.80-0.85 V more negative than a reference silver/silver
chloride electrode in the water nearby, then the hull is
adequately protected. Current density of the order of
20-100 mA/m2 is usually sufficient on a painted hull to
reverse any corrosion current and cease further metal
corrosion. Current density necessarily increases for a
poorly painted hull and therefore cathodic protection
should be regarded as an additional protection to painting
and by no means a substitute.

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Sacrificial Anode
Two means of cathodic protection are in general use on
ships – the sacrificial anode type and the impressed current type.
The sacrificial anode type of cathodic protection uses
metals such as aluminum and zinc which form the anode
of a corrosion cell in preference to steel. As a
consequence, these sacrificial anodes are gradually eaten
away and require replacement after a period of time. The
impressed current system provides the electrical potential
difference from the ship’s power supply through an anode
of a long-life highly corrosion-resistant material such as
platinised titanium.

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Ship classification societies have established standard
calculation forms for hull loads, strength requirements,
the thickness of hull plating and reinforcing stiffeners,
girders, and other structures. These methods often give a
quick and dirty way to estimate strength requirements for
any given ship. Almost always those methods will give
conservative, or stronger than precisely required,
strength values. However, they provide a detailed starting
point for analyzing a given ship's structure and whether it
meets industry common standards or not.
Metal Fatigue (Deformation and Fractures) 
Standard Rules

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Modern ships are, almost without exception, built of
Steel. Generally this is fairly standard steel with Yield
strength of around 32,000 to 36,000 PSI, and Tensile
strength or Ultimate Tensile Strength (UTS) over 50,000 PSI.
Shipbuilders today use steels which have good corrosion
resistance when exposed to seawater, and which do not
get Brittle at low temperatures (below freezing) since
many ships are at sea during cold storms in wintertime,
and some older ship steels which were not tough enough
at low temperature caused ships to crack in half and
sink during WW II in the Atlantic.
The benchmark steel grade is ABS A, specified by the
American Bureau of Shipping. This steel has a yield
strength of at least 34,000 PSI, ultimate tensile strength
of 58 to 71,000 PSI, must elongate at least 19% in an 8
inch long specimen before fracturing and 22% in a 2 inch
long specimen.
Material Response

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A safety factor above the yield strength has to be
applied, since steel regularly pushed to its yield strength
will suffer from metal fatigue. Steels typically have a
fatigue limit, below which any quantity of stress load cycles
will not cause metal fatigue and cracks / failures. Ship
design criteria generally assume that all normal loads on
the ship, times a moderate safety factor, should be below
the fatigue limit for the steel used in their construction. It
is wise to assume that the ship will regularly operate fully
loaded, in heavy weather and strong waves, and that it
will encounter its maximum normal design operating
conditions many times over its lifetime.

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Designing underneath the fatigue limit coincidentally
and beneficially gives large (factor of up to 6 or more)
total safety factors from normal maximum operating
loads to ultimate tensile failure of the structure. But
those large ultimate safety margins are not the intent:
the intent is that the basic operational stress and strain
on the ship, throughout its intended service life, should
not cause serious fatigue cracks in the structure. Very
few ships ever see ultimate load conditions anywhere
near their gross failure limits. It is likely that, without
fatigue concerns, ship strength requirements would be
somewhat lower.

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Nmlc ef4 module1 day03 corrosion and preventions